In video image cameras that utilize charge-coupled devices (CCDs) as the mechanism for transferring a detected signal out of a sensor scan array in the camera, it is often necessary to process the raw video signal developed by the CCD camera before useful image information can be extracted. One of the steps in processing the raw video signal is to remove a dark level DC bias from the raw video signal. To understand this process, it is necessary to understand something about both the way in which the raw video signal is generated by the CCD camera and the manner in which the video signal will be utilized once it has been processed.
Typically, the information portion of the video output signal generated by a CCD camera consists of a voltage signal referred to as the pedestal. During a first portion of the camera scan time, light striking a photosite in the scan array will discharge or decrease a reference charge level to which the photosite has been precharged. During a second portion of the camera scan time, the charge level remaining at the photosite is transferred out of the scan array using the charge-coupled device read out mechanism. Thus, the pedestal is the difference between the reference charge level or dark level and the charge level that remains at the end of the first portion of the camera scan time.
The reference charge level is sometimes referred to as a black level or dark level voltage value because if no light is received at a photosite during the first portion of the camera scan time, then the charge level remaining at that photosite which represent a dark or no light condition is equal to the reference charge level to which the photosite was initially charged. A fully discharged photosite, on the other hand, is said to be in a state of saturation; however, even when in the saturation state a certain amount of charge will remain at the photosite. The differences in charge levels contained in the pedestal signal are representative of differences in light received by different photosites from different locations in the focal plane of the CCD camera and will cover a defined range of voltage values for a particular CCD camera. In the raw video output signal from the CCD camera, this defined range of voltage values is riding on a DC bias or DC offset voltage that is equivalent to the dark level voltage value a DC bias or dark level offset on which the useful video information rides. Depending upon whether the reference voltage to which the photosites are initially charged is positive or negative, this DC bias may be either a positive or negative value. In other words, the useful range of information in the video signal representative of the amount of light received by a photosite during one camera scan time, such a signal varying between 0 V and 1 V, for example, is positioned on top of the DC bias signal of -4 V, for example. As a result, the voltage value of the raw video signal in this example would vary between -4 V and -3 V.
In many applications, once a raw video output signal has been produced by the CCD camera, the video output signal is then supplied to some type of analog-to-digital (A/D) converter so as to generate a stream of digital bits representative of the video image seen by the CCD camera. In high fidelity applications, such as vision inspection systems or high quality image reproduction, it is desirable to maintain both the information content of both the analog and digital versions of the video signal as pure as possible without introducing errors or noise into the signal.
Because the accuracy of A/D converters is a function of the voltage range of the input signal, it is preferable to remove the dark level value as a DC bias from the video output signal before the analog-to-digital conversion is performed. If the dark level value is not removed, then a large portion of the accuracy of the A/D converter is lost in converting the DC bias value represented by the dark level value, rather than in obtaining a higher resolution conversion of the voltage differences from the dark level value which actually represent the useful information in the video output signal. As a result, some type of dark level compensation circuitry is usually included within the video preprocessing circuit that is typically part of the CCD camera so as to compensate for the dark level value before an A/D conversion is performed.
The most prevalent type of dark level compensation circuitry is a DC restoration circuit that reinserts the DC component of the raw video signal after the DC bias has been removed by way of an AC coupling in the form of a high pass capacitive filter. Examples of DC restoration circuits for AC coupled video signals are shown in U.S. Pat. Nos. 3,976,836, 4,549,215, 4,712,010, 4,713,694, and 4,963,963. The problem with DC restoration circuits is that low frequency information in the video signal is often lost to the low pass capacitive filter that AC couples the raw video signal to the preprocessing circuit.
Another possible type of dark level compensation circuit is a circuit which subtracts a fixed reference voltage from the raw video signal. While this solution is DC coupled and hence does not lose low frequency information in the video signal, it necessarily introduces a significant amount of error into the raw video signal because the dark level value will never be a constant fixed value that is identical to the fixed reference voltage which is subtracted from the raw video signal. Drift and variation in the dark level voltage value will be caused by changes in temperature, pixel clock frequency, and ambient conditions, as well as from camera to camera. For high fidelity applications where the introduction of any noise or error into the video signal is undesirable, this solution is simply not viable.
A variation on the circuit which subtracts a fixed reference voltage is shown in U.S. Pat. No. 5,010,408 where the reference voltage is a sampled voltage. In this case, the subtraction is accomplished using a differential amplifier which is connected in an open loop to receive the raw video signal. As with the previously discussed technique, this circuit does not compensate for offset variation the CCD camera. In addition, any amplification of the video signal following the differential amplifier will reintroduce a variable offset voltage into the raw video signal.
Another type of DC coupled preprocessing circuit sometimes used for video signals is a peaking control circuit, such as shown in U.S. Pat. Nos. 4,388,648 and 4,635,118. U.S. Pat. No. 4,994,917 shows another type of preprocessing circuit that includes a clipping circuit. Both peaking control circuits and clipping circuit, by their very nature, necessarily alter the information content of the video signal. As a result, these types of circuit are not used in high fidelity applications where the objective is to preserve the original information content of the video signal.
Still another type of dark level compensation uses digital correction values stored in a memory to correct the video signal after it has been converted to a digital video signal by an A/D conversion process. Examples of digital dark level correction include U.S. Pat. Nos. 5,086,343 and 5,181,118. The problem with compensating for the dark level bias after the A/D conversion process is that the range of the A/D converter is now used up by the large DC bias voltage. In addition, the error of any gain introduced to the raw video signal before the A/D converter cannot be compensated for after the A/D conversion process.
To solve the later problem of not being able to compensate for errors in the gain of the raw video signal prior to the A/D conversion, several dark level compensation circuits have included the A/D converter within a feedback loop. The output of the A/D converter is used by a control circuit to supply feedback to an amplifier or integrator that is included in the circuit before the A/D converter. Examples of this type of hybrid feedback loop for controlling dark level offset include U.S. Pat. Nos. 4,525,741, 5,101,271 and 5,105,276 and PCT Publ. No. WO/89/10037. While this solution addresses the problems of not being able to compensate for gain errors introduced to the raw video signal, it still does not overcome the problem of using up all of the range of the A/D converter. In addition, this type of hybrid circuit cannot compensate for errors inherent in the A/D converter.
Although existing dark level compensation circuits for a CCD camera are adequate for many video signal preprocessing requirements, these circuits have proven inadequate in applications which require high fidelity of the original raw video signal generated by the CCD camera. Consequently, a dark level compensation circuit which can remove a dark level DC bias from a raw analog video signal generated by a CCD camera without losing signal information or introducing noise into the video signal would be greatly appreciated.